Automated testing for cellular telephone system including emergency positioning

ABSTRACT

A method for automatically testing cellular telephone equipment includes a method for determining a geographic location of a mobile unit. The system includes monitoring sites located, e.g., at high elevations, so that each monitoring site electronically covers a geographical area including several cellular telephone base stations. Locations of mobile cellular stations, especially of such stations placing emergency 911 calls, are determined by comparing signal time-of-reception and other observable signal parameters at a combination of three cell sites and/or monitoring sites. Testing functions include transmitting gradually increasing power levels on a frequency assigned to a particular base station to determine the power level required to acquire service from that base station. Periodic repetitions are monitored over time to indicate any changes or degradation in performance. A scanner scans a designated group of control-channel frequencies, selecting active channels for measurement of transmission parameters and detection of anomalous transmission characteristics on any particular channel.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] Not Applicable.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to methods and apparatus for determining ageographic position of a mobile telephone station, in combination withautomated testing of cellular telephone equipment; and in particular tosuch methods and apparatus including elevated monitoring sites foractive and passive testing of cellular base stations and mobilestations.

[0004] 2. Description of the Related Art

[0005] Self-testing capabilities of cellular telephone sites currentlyincludes test functions such as monitoring antenna performance bymeasuring the voltage standing wave ratio (VSWR), and monitoring sitecontroller malfunctions and environmental conditions such as airconditioning and power failures. Generally, self-testing of electronicequipment detects relatively major malfunctions such as failures ofindividual radios or entire cell sites.

[0006] Currently, self-testing does not include remote monitoring oftransmitted power levels to ensure that the entire signal path throughthe antenna is working properly and that the antenna is efficientlyradiating the power delivered to it. Use of test mobiles at a site isknown, but these are used to detect major failures, i.e., a problem isdetected if the test mobiles do not respond when called. What is neededis an automated system which tests a complete end-to-end signal path,and which tests all the channels (frequencies, time slots, or PN codes)assigned to a particular cell site so that the entire system, includingaudio paths, is tested. Such a test system would not only detectequipment failures, but also detect more subtle changes in timingparameters, operational parameters, and system settings.

[0007] Such a testing system, properly designed, can also determine thelocation of cellular mobile stations in emergency situations. Thefamiliar “911” is widely used as an emergency telephone number; 911requests are relayed to the proper emergency-services department forresponse. The effectiveness of emergency services depends, of course, onemergency personnel being able to get to the caller without undue delay.To this end, the U.S. government has recently promulgated regulationsrequiring that providers of cellular telephone service be able to supplyinformation on the position of a mobile station making an emergencycall.

[0008] A problem with responding to requests for emergency assistance isthat persons using mobile telephones often are unable to give theirexact location when making an emergency call. A caller who happens to bein unfamiliar terrain does not know local landmarks, and may not knowthe name of the street or road on which he is driving. This lack ofinformation regarding the caller's location hampers and delays effortsto get emergency assistance to the caller, whether the request is formedical assistance, the police, or roadside repairs.

[0009] The geographic size and shape of each cell in a cellulartelephone network is largely determined by the coverage oftransmit/receive antennas located at the central cell site, and by thesurrounding terrain. In open country, cells are substantially circular,with overlapping borders. Where buildings or uneven topography blockradio-frequency transmissions, the cells may be quite irregular inshape.

[0010] A mobile telephone station is handed off from cell to cell basedon the received signal strength and signal quality at each cell site,with the call being routed through the cell receiving or providing thebest signal. In a metropolitan area, a number of cells may be capable ofreceiving a 911 emergency call from a particular mobile station.Triangulation, using the signal time-of-arrival at three or more sites,angle of arrival, or some combination of observable signal parameters,may be used to determine the position of such a caller. In rural areas,however, cell sites are generally too widely spaced for triangulation tobe possible. This gives rise to situations in which a caller makes anemergency 911 call, is asked his location by the dispatcher, and isunable to give accurate information. Emergency services are thusdelayed, sometimes with serious consequences. What is needed, thereforeis a system combining automated testing functions with the ability todetermine a geographic position of a mobile station.

SUMMARY OF THE INVENTION

[0011] Parameters of cellular system performance, both base stations andmobile stations, are measured at automated monitoring sites. At leastsome of the monitoring sites are located at high-elevation sites to givea large-radius radio horizon, and/or at existing cell sites. Amicroprocessor, under either local or remote control, is placed at eachmonitoring site. The microprocessor controls a number of “test mobiles;”the test mobiles are interconnected to transmit/receive antennas. Poweroutput of the test mobiles can be varied by the control microprocessor;the test mobiles are also capable of transmitting without the usual callset-up process, “creatively” violating their usual protocol in order totest system functions.

[0012] By slowly increasing or decreasing transmitted power on aparticular frequency, the monitoring site can periodically test thepower level required to acquire service on each channel served by a cellsite, and the power setpoints or “dynamic power control” properties thesite exhibits. Hand-off performance between cells is tested byincreasing power on one test mobile and decreasing it on another at adifferent location, and recording the levels at which the hand-offoccurs. Advanced protocols such as IS-136 and IS-95 require the testmobile to manipulate its reporting of signal parameters received from aserving site, but handoff testing may be accomplished in a conceptuallysimilar way.

[0013] Recording time-of-arrival at several monitoring locations of asignal from a mobile station making an emergency 911call provides datafor determining a position solution for the mobile station. The presentinvention determines TDOA (time difference of arrival) by separating areceived signal into its component frequencies, and plotting phasedifferences for individual frequencies at different receiving sites,either cell sites or monitoring sites. A slope of the phase-shift plotis proportional to the TDOA at different sites. The arrival times thusdetermined are used to plot a geographic location of the mobile stationgenerating the call.

[0014] Data processing for the positioning functioning is done by acentral E-911 computer, which may also be linked to the mobile telephoneswitching office (MTSO) computer. Occasionally data will be availablefrom only the serving cell site and one other site, in which case themobile's position can be narrowed to two possible choices. Comparison ofthe two locations to a map showing locations of roads, etc., can ofteneliminate one of the positions. Data from a combination of three sites,either cell sites or monitoring-only sites, using a time-of-arrivalalgorithm, will give a specific geographic location of the mobilestation.

[0015] The same transmitter-locating technology may also be used for“value-added” services such as fleet vehicle tracking, non-emergencymotorist assistance, etc., limited only by the imaginations of serviceproviders. Thus for the purposes of this application, “911calls” mayinclude non-emergency calls or mobile registration actions which alsoare used to generate location fixes.

[0016] Based on the above, it is an object of this invention to providea method by which operational parameters of cellular telephone sites maybe monitored on an ongoing basis.

[0017] Another object is to provide a combination of a testing andmonitoring system which further uses time data and/or other observablesignal parameters collected by a monitoring site and by cellular basestations to automatically provide an accurate geographic location of amobile station placing an emergency 911call, or to monitor location ofcertain mobile stations on a continuing basis, utilizing routinetransmissions including registration and/or non-emergency transmissions.

[0018] A further object of the invention is to provide such a methodwhich is automated, and which periodically tests the complete signalpath or channel for each cellular frequency served by a particular site.

[0019] Another object is to provide a method by which several cellularsites may be monitored and tested by a single monitoring site located ata high elevation.

[0020] The specific nature of the invention, as well as other objects,uses, and advantages thereof, will clearly appear from the followingdescription and from the accompanying drawings, the different views ofwhich are not necessarily scale drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a schematic of a geographical area including threemonitoring sites and their respective radio horizons, within which areincluded several cellular base stations and a variable number ofcellular mobile stations.

[0022]FIG. 2 is a schematic view of mobile telephone stations within anurban area, in which cell sites are placed more closely together.

[0023]FIG. 3 is a schematic view of mobile stations in a rural area, inwhich cell sites are placed as far apart as practicable, and some cellsite coverage gaps exist.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Referring now to the drawings, FIG. 1 shows a map view of ageographical area including three remote cellular monitoring sites 10.As shown in the drawing, the radio horizons of the monitoring sites aremuch larger than the radio horizons of the cellular base stations 12.Located on tall towers or other suitably elevated locations such asmountains, tall buildings, etc., the monitoring sites are placed toreceive RF transmissions from a number of base stations. Cellulartelephone base stations are commonly referred to as “cell sites.” Themonitoring sites also receive transmissions from cellular telephonemobile stations 14 within their radio horizon. A mobile station 14 isdefined as a cellular telephone station, either mobile, portable, orhand-held, used by a person on foot or in any type of vehicle, includingground vehicles, boats and other marine vessels, and aircraft. “Cellulartelephone system,” as used herein, is defined as including PCS systems;although PCS frequencies are generally different from cellularfrequencies, some PCS and cellular telephone systems are transparent toone another.

[0025] The monitoring sites 10 may be thought of as data collectioncenters; they operate both in an active mode and in a passive mode,collecting a variety of data regarding cell site performance, and alsodata which is used to determine position solutions for particular mobilestations.

[0026] Referring to FIG. 2, a mobile station 14 placing a call usuallyis within range of several cell sites 12 in an urban area. This isfrequently true in urban areas, but in rural areas (FIG. 3) a signalfrom a mobile station will be received at fewer cell sites, perhaps onlyone. A call from a mobile station is carried by a particular cell sitebased on the received signal strength at/from the various cell sitesreceiving the signal (or on other signal quality measures, dependingupon the protocol used); the call will be routed through the site(s)receiving the strongest or best quality signal. Of course, changes inthe mobile station's position will cause the mobile station's signallevel and signal quality to change at the various cell sites withinrange. When the signal level at (or from) another cell site becomesstronger or of better quality than that at the original cell site, therouting of the call is switched or “handed off” to the site receiving orproviding the stronger or higher-quality signal. Such a hand-off isautomatic and so brief that a caller is usually unaware when it happens.The preceding description applies to cellular systems in general,whether the mobile units are in use by pedestrians, or aboard groundvehicles, watercraft, or aircraft.

[0027] One function of the automated testing system is to measure thesignal level required to acquire service from a particular cell site 12.To perform this test a monitoring site 10 goes “active,” transmitting ona cellular channel control channel assigned to a cell site 12 withinrange of the monitoring site. The power level of the transmitted signalis initially low enough that it is near or below the noise threshold ofthe cell site equipment, and thus is not recognized by the cell site. Asthe power level is gradually increased, the signal reaches a level whichis detected by the cell site under test, which then begins the processof establishing the call. The power level required to acquire service isrecorded; the test is repeated periodically, so that comparison of testdata indicates any change, trend, or degradation in equipmentperformance. Also, of course, major failures such as a lack of responseby the cell site on one or more channels are immediately obvious.

[0028] Another function of the system is to test operation of hand-offsbetween cell sites. A monitoring site can be instructed to establish acall through a particular cell site. With the call established, thetransmit power level from one monitoring site is decreased while thetransmit power level is increased at the other monitoring site. Inadvanced protocols, the monitoring sites may also provide increasinglybiased or offset measures of signal quality. The effect is the same asthough a mobile station were traveling through the cellular network,with its RF link to one cell site weakening while growing stronger andof better quality to the other. When the system performs a hand-off, thetime of hand-off is recorded along with the power levels (and applicablesignal quality reports) being transmitted to each cell site. The systemmethodically tests channels in use at each cell site, and may performseveral back-and-forth handoffs during each channel test. Such testinginitially establishes a baseline of data for comparison to future tests;as described above, comparison of periodic hand-off tests will indicateany changes in cell-site performance parameters. By handing the callback and forth between the cell sites, any bias in the hand-offprocedure can be detected and measured, as well as the speed of responseby each cell site.

[0029] In a passive mode, a monitoring unit at a cell site 12 scanscellular frequencies and/or logical “channels”, looking for activity. Areceiver is programmed to detect cellular channels for cell sites withinthe monitoring site's range; the receiver may be part of, or integratedinto, the test mobiles in the monitoring unit. Adjacent or nearby cellsmay not use the same cellular channels because such use would causeinterference. Many cell sites may be assigned the same cellularchannels, but those using the same channels must not be close enough tocause interference with each other's calls. When the test system detectsa received signal on a particular channel, the computer controllerdetermines whether a call is in progress on that forward channellocally. Lack of a local call indicates that some nearby cell is usingthat channel, increasing chances for interference. Data collected in themode just described is recorded for analysis.

[0030] Another function of the system is analysis of data to determinethe location of a mobile station. FIG. 2 illustrates mobile telephoneunits placing 911calls within the cellular telephone system of an urbanarea. Given the density of cell sites, such a call is often received atseveral cell sites; a known relative-time-of-signal-arrival procedurewill be used to time-stamp the call. The same procedure is used tocollect time information from other cell sites which receive the call.Using the relative time-of-arrival information, a triangulationalgorithm is used to determine a position of the mobile station fromwhich the call is placed. In some regions shown in FIG. 2, monitoringsites not colocated with cell sites provide additional time-of-arrivaldata points necessary to provide a unique position solution or toenhance accuracy.

[0031]FIG. 3 is a schematic of a mobile telephone unit in a rural area.Wider spacing of cell sites precludes triangulation by signaltime-of-arrival as described above without the use on non-colocatedmonitoring sites 10, because the call is not received at a sufficientnumber of cell sites. According to the invention, however, the call willbe received at one or more monitoring sites 10 in addition to one ormore cell sites 12. In some regions of FIG. 3, these monitoring sitesprovide the majority of the data points needed for a unique positiondetermination. Each monitor site is configured to monitor controlchannels accessible by a mobile unit within the radio horizon of themonitor site. Time utilization of reverse control channels isstatistically low, therefore position determination (time of arrival)data may be received by monitoring stations 10 even where the monitoringstation 10 receives signals from regions containing more than one cellsite 12 reusing the same control channel(s). Because the cellulartelephone system in the United States is structured so that each area isserved by two cellular service providers and possibly several PCSproviders, the available control channels may include those of more thanone provider, with potential sharing of monitoring resources.

[0032] To locate the position of a mobile station, call access attemptsare monitored. A “call access attempt” by a mobile station, as usedherein, includes a call origination, a response to a call, or aregistration. Detection of preselected dialled numbers such as theemergency 911number, or preselected user identity numbers prompt themonitor controller to mark the arrival time of the signal, the ESN/MIN(Electronic Serial Number/Mobile Identification Number) of the callingunit, and the channel on which the call is received. Other informationrecorded includes the DCC or other parameters which identify the sitebeing accessed, and observable parameters such as angle of arrival. Themonitor controller then contacts a central computer, an E-911 computerdesignated to receive location information; this contact may be adial-up call (either land-line or cellular), a “connectionless” networklink, or via a dedicated circuit. With the contact established, theESN/MIN, time information, and observable signal parameters will bedownloaded to the E-911computer. The information thus transmitted iscombined, by the E-911computer, with the similar information received bythe serving cell site and/or other monitoring sites.

[0033] Using a time-difference-of-arrival algorithm, the E-911computerdetermines a geographic location of the mobile station from which thecall was placed. In an alternate embodiment, the information processingto determine a geographic location is performed by the computercontroller for the cellular telephone system; that is, the“E-911computer” is but a software package in another computer.

[0034] The present invention uses a frequency domain approach todetermine the signal time of arrival at various monitoring sites. Thisdiffers from the traditional time domain method, which involvesperforming a cross correlation on signals received a two or more sites.Correlation is done directly by the correlation integral, or by theFourier transform method. (In the “Fourier Transform Method:, the twotime domain functions are transformed to the frequency domain, thenmultiplied (Hermetian product) together, and the product is inversetransformed.) In either case (they are mathematically equivalent), across correlation function is generated; the position of the peak ormaximum of the cross correlation function is related to the time shiftbetween the two functions.

[0035] In this invention a frequency domain approach is used, in whichthe received signals from two different locations are decomposed intotheir constituent frequencies. The phase functions for the two signalsare found. The difference in these phase functions, when expressed as afunction of frequency, will ideally be a linear equation whose slope isdirectly proportional to the time difference of arrival. Likewise the“y” intercept will be the relative frequency error between the twosignals. Since there will likely be noise present, a linear regressionmay be performed to find the slope and intercept. Alternatively, aderivative of the phase function may be found. Ideally, it will have aconstant valued function of frequency that may be smoothed by filtering.This phase derivative approach offers an advantage of avoiding thediscontinuities that accompany the phase unwrapping associated withfinding the phase function.

[0036] Advantages of the frequency approach just described include: 1)avoiding the computation of the cross correlation function and thesubsequent peak search; 2) allowing direct frequency domain filtering tooptimize signal-to-noise ratio, i.e., the signal power as a function offrequency may be used to weigh the filtering; 3) a direct yield of theTDOA; 4) utilization of the efficient fast Fourier transform; and 5)using an easy way to avoid the phase unwrapping problem associated withfinding phase shifts.

[0037] It is understood that unique determination of a mobile station'sgeographic location requires data from three or more sites. In a ruralarea, data may be available only from a monitoring site and one basestation. Most location algorithms, given data from two sites, willprovide two possible locations for the mobile station placing the call.In most cases, however, these possible locations can be narrowed to onewhen the location data is superimposed on a map. If one locationcoincides with a road or a populated area while the other does not, theformer is the most likely location of the mobile station.

[0038] The restrictive description and drawings of the specific examplesabove do not point out what an infringement of this patent would be, butare to enable one skilled in the art to make and use the invention.Various modifications can be made in the construction, material,arrangement, and operation, and still be within the scope of ourinvention. The limits of the invention and the bounds of the patentprotection are measured by and defined in the following claims.

Subject matter claimed for protection: We claim as our invention:
 1. Amethod for determining a geographic position of a mobile telephonestation in combination with automatic testing of cellular telephoneequipment, comprising the following steps: locating a plurality ofmonitoring sites so that each said monitoring site receivescellular-frequency transmissions within a geographical area includingone or more cellular telephone base stations, electronically connectingsaid monitoring sites to a computer controller, monitoringcellular-frequency control channels accessible by a cellular telephonemobile station within a radio horizon of said monitoring site,measuring, at said monitoring site, signal parameters of monitoredtransmissions from said mobile stations and from said base stations, anddetermining a position of a mobile telephone station by plotting phasedifferences versus frequency for a plurality of frequencies received attwo or more monitoring sites from said mobile telephone station.
 2. Themethod as described in claim 1, wherein one or more said monitoringsites are colocated with cellular telephone base stations.
 3. The methodas described in claim 1, wherein the step of determining a position of amobile telephone station further comprises: decomposing a signalreceived from a mobile telephone station into said signal's componentfrequencies, determining a magnitude and a phase for a plurality of saidcomponent frequencies, determining, versus frequency, differences inphases for a plurality of individual frequencies received at two or moremonitoring sites, measuring a slope of a phase-vs-frequency relationshipfor a plurality of frequencies, said slope being directly proportionalto a relative time of arrival of said signal at differing monitoringsites, and computing from said time of arrival at different monitoringsites a geographic location estimate for said mobile telephone station.4. The method as described in claim 1, wherein the step of locating aplurality of monitoring sites further comprises: locating at least onesaid monitoring site at a sufficiently high elevation so that saidmonitoring site receives cellular-frequency transmissions within ageographical area including two or more cellular telephone basestations.
 5. The method as described in claim 1, wherein the step ofmonitoring cellular-frequency control channels further comprises:scanning a pre-determined group of control channel frequencies using anautomated scanner.
 6. The method as described in claim 1, furthercomprising the following steps: measuring a power level required toaccess service from a particular base station by transmitting from onesaid monitoring site, at a gradually increasing power level, on acontrol channel assigned to said base station, recording a transmittedpower level at which said base station responds to a transmission fromsaid monitoring site, repeating the measuring step and the recordingstep from time to time, and comparing recorded transmitted power levelsfrom repeated measuring steps to detect any change in a power levelrequired to access service from said base station.
 7. The method asdescribed in claim 1, further comprising the following steps: measuringdynamic power-control performance of a particular cell site byestablishing a test call between a monitoring site and said cell site,transmitting from said monitoring site at different power levels duringsaid test call, recording a power level transmitted from said monitoringsite at which said cell site initiates a power-adjustment command, andrepeating said transmitting step and said recording step for differentlevels of power transmitted from said monitoring site.
 8. The method asdescribed in claim 1, further comprising the following steps: measuringa power level required to access service from a particular base stationby transmitting from one said monitoring site, at a gradually increasingpower level, on a control channel assigned to said base station,recording a transmitted power level at which said base station respondsto a transmission from said monitoring site, repeating the measuringstep and the recording step from time to time, and comparing recordedtransmitted power levels to detect any change in said power levelrequired to access service from said base station.
 9. A method fordetermining a geographic position of a mobile telephone station incombination with automatic testing of cellular telephone equipment,comprising the following steps: placing a plurality of monitoring sitesat sufficiently high elevations so that each said monitoring sitereceives cellular-frequency transmissions within a geographical areaincluding one or more cellular telephone base stations, electronicallyconnecting each said monitoring site to a computer controller,monitoring cellular-frequency control channels accessible by a cellulartelephone mobile station within a radio horizon of said monitoring site,detecting a call access attempt from said mobile station, recording atime at which said call access attempt was received at said monitoringsite, recording an electronic identity of said mobile station makingsaid call access attempt, determining a base station through which saidcall access attempt is routed, comparing said time at which said callaccess attempt was received at said monitoring site to a time at whichsaid emergency call was received at said base station, and comparing therecorded times to determine a position estimate of said mobile station.10. The method as described in claim 9, wherein said call access attemptincludes dialled digits “911”.
 11. The method as described in claim 9,wherein the step of recording said electronic identity of said mobilestation includes recording other observable signal parameters from saidmobile station.
 12. The method as described in claim 9, wherein the stepof recording said electronic identity of said mobile station includesrecording a serial number (ESN) of said mobile station.
 13. The methodas described in claim 9, wherein the step of recording said electronicidentity of said mobile station includes recording a mobileidentification number (MIN) of said mobile station.
 14. The method asdescribed in claim 9, further comprising the following steps: comparingsaid recorded times with a recorded time at which said call accessattempt was received at a third cellular facility, and calculating fromthe recorded times at which said call access attempt was received atsaid monitoring site and at said base station and at said third cellularfacility station to determine an exact location of said mobile station.15. The method as described in claim 9, further comprising the followingsteps: comparing said possible position of said mobile cellulartelephone station with information on a geographic map to determine amost likely position of said mobile station.
 16. A method for automatictesting to monitor performance of cellular telephone equipment by acombination of passive signal monitoring and active testing, comprisingthe following steps: accessing a cellular telephone system via an RFlink, testing, via one or more monitoring sites, signal qualityparameters of transmissions from cellular mobile stations and cellularbase stations, controlling said testing with a test and measurementcontrol computer, and reporting from said monitoring sites variations insaid signal quality parameters outside predetermined limits.
 17. Themethod as described in claim 16, wherein said monitoring site includeson-site test equipment at a cellular telephone base station.